A light emitting diode (LED) is a solid-state semiconductor element including at least a p-n junction. The p-n junction is formed between a p-type and an n-type semiconductor layers. When the p-n junction receives a suitable forward voltage, the holes of the p-type semiconductor layer and the electrons of the n-type semiconductor layer are combined to emit light. Generally, the region emitting light is called a light-emitting region.
The characteristics of LEDs are small dimension, high lighting efficiency, long lifetime, quick reaction, high reliability, and great chromaticity, so LEDs have been applied widely in electronic devices, motor, signboard, and traffic signals. With its full color spectrum, LED has gradually replaced conventional lighting apparatus such as fluorescent lamps and incandescent lamps.
Generally, a LED chip is utilized with a wavelength-converted material such as phosphor to generate white light. The wavelength-converted material can emit yellow light, green light, or red light after receiving blue light from the LED chip, and then blue light and yellow light, green light, or red light can be mixed to generate white light. Because the emitting direction of light is omnidirectional to ensure that light from an LED chip can pass the wavelength-converted material and be mixed to generate the desired light, the wavelength-converted material has to totally cover the positions where light emits. If the wavelength-converted material does not totally cover where light emits out of the LED chip, a portion of light such as side-emitting light does not pass the wavelength-converted material and the wavelength converting efficiency is reduced. On the other hand, although the wavelength-converted material totally covering the LED chip can increase wavelength-converting efficiency, it causes some problems like poor heat dissipation.
It is not easy to cover the LED chip uniformly with the wavelength-converted material. And because thicker wavelength-converted material receives more light than thinner wavelength converted material does, the chromaticity of light is different when the omnidirectionally emitted light passes the wavelength-converted materials of different thickness. Although there are a lot of complicated methods such as electrophoresis which is able to coat the wavelength-converted material on the LED chip uniformly, the manufacturing cost is higher or the yield is lower. In addition, the wavelength-converted material normally has to be formed with an adhesive material like glue so it is unavoidable that much of light is absorbed by the adhesive material and the wavelength-converting efficiency is therefore damaged.